Acrylic fibers based on acrylonitrile have heretofore been used extensively in the apparel market because of their superior properties such as outstanding resistance to sunlight and dyeability. However, acrylic fibers are low in mechanical strength compared with other synthetic fibers such as nylon and polyesters and little use has been made of them in industrial materials. Therefore, many attempts are proposed to produce acrylic fibers that have mechanical characteristics that would make them suitable for use in industrial materials. Acrylic fibers can be oxidized and carbonized to make carbon fibers and because of their high strength and modulus, carbon fibers are receiving considerable attention as reinforcements in composite materials. Since the physical properties of carbon fibers are determined by the characteristics of the starting acrylic fibers, active efforts are also being made to modify them to make suitable precursors for carbon fibers.
According to Japanese Patent Application (OPI) No. 61-152811(1986) (the term "OPI" as used herein means a "published unexamined Japanese patent application") or Japanese Patent Application (OPI) No. 61-6160(1986) (corresponding to European Patent Application 165,372A), which describes the result of such an attempt, a spinning solution of an acrylonitrile based polymer having an intrinsic viscosity of 2.5 to 3.6 is spun by the dryjet wet spinning process, and the resulting coagulated filaments are stretched to a draw ratio of 2 to 10 either after or during washing, the filaments being then dried, collapsed and stretched to a draw ratio of 1.2 to 5 at 180.degree. to 240.degree. C. under the action of dry heat so as to produce an acrylic fiber having a strength of 13.5 g/d and a modulus of 235 g/d. Another method for producing an acrylic fiber having improved mechanical characteristics has been proposed in Japanese Patent Application (OPI) No. 61-97415(1986) (corresponding to U.S. Pat. No. 4,658,004) in which an acrylonitrile based polymer having a Mw/M n ratio of not more than 7.0 and a weight-average molecular weight (Mw) of at least 4.times.10.sup.5 is dissolved in a suitable solvent with defoaming under vacuum, and the resulting spinning solution is spun into filaments, which are then coagulated, stretched in multiple stages under progressively increasing temperatures, and dried under tension at temperatures of not higher than 130.degree. C. According to this method, an acrylic fiber having a tensile strength of 18.8 g/d and a sonic modulus of 3.2.times.10.sup.11 dyn/cm.sup.2 is produced.
Further, Japanese Patent Application (OPI) No. 59-199809(1984) (corresponding to U.S. Pat. Nos. 4,535,027 and 4,659,529) describes a method for producing an acrylic fiber having high strength in which an acrylonitrile based polymer is dissolved in an aqueous solution of rhodanate and is spun.
However, these methods have their own problems. First, the method described in Japanese Patent Application (OPI) No. 61-152811(1986) or No. 61-6160(1986) is applicable only to acrylic polymers having a very narrow range of molecular weight (an intrinsic viscosity of 2.5 to 3.6 is equivalent to a molecular weight of about 2.8.times.10.sup.5 to about 3.8.times.10.sup.5) and in order to achieve consistent spinning from a polymer having an intrinsic viscosity of more than 3.6, the polymer concentration of the spinning solution must be lowered to about 10 to 15 wt %. However, if a spinning solution having such a low polymer concentration is used, a large quantity of solvent must be removed after the formation of coagulated filaments and this increases the chance of the fiber structure becoming porous and opaque. In addition, if such a spinning solution is subjected to the dryjet wet spinning process which is considered to be most appropriate for the purpose of making high-strength acrylic fibers, individual single years are highly likely to fuse together during the stretching or drying step, thereby making it difficult to manufacture desired multifilaments. These problems associated with the increase in the molecular weight of acrylic polymers are specifically mentioned in Japanese Patent Application (OPI) No. 61-152811(1986).
The method described in Japanese Patent Application (OPI) No. 61-97415(1986) starts with an acrylic polymer having a weight-average molecular weight of at least 4.times.10.sup.5. A polymer having such a high degree of polymerization can only be produced by performing aqueous suspension polymerization in the presence of a dispersion stabilizer such as polyvinyl alcohol. Furthermore, a solution of such a polymer is so viscous that considerable difficulty is involved in defoaming it. In addition, since ease of spinning is directly influenced by the viscosity of a polymer solution, the polymer concentration of the spinning solution used in Japanese Patent Application (OPI) No. 61-97415(1986) has to be lowered compared with ordinary spinning solutions, but then, as already mentioned, the decrease in polymer concentration causes various problems and is not considered an industrially feasible method in view of the fiber quality obtainable and the consistency of spinning operations. As a further problem, in order to have desired physical properties developable in the acrylic fiber prepared from such a spinning solution of low polymer concentration, the fiber must be stretched to a very high draw ratio. As shown in Japanese Patent Application (OPI) No. 61-97415(1986), a draw ratio as high as 36 is necessary in order to produce an acrylic fiber having a tensile strength of 17.2 g/d. But stretching by such a high draw ratio generally causes unevenness in fiber fineness and only fibers of low quality will result. It is also well known that as acrylic fibers are stretched, fibrillation progresses and the acrylic fibers which are inherently low in wear resistance, will become even less wear-resistant. The acrylic fiber attained by the spinning technique proposed by Japanese Patent Application (OPI) No. 61-97415(1986) may develop comparatively satisfactory strength, but other mechanical properties such as tensile modulus and knot strength cannot be attained in a balanced way. Therefore, this acrylic fiber does not afford fiber characteristics as good as aromatic polyamide fibers, i.e., Aramid fibers which, as typified by Du Pont's "Kevlar" having a tensile strength of up to 20 g/d, are highly adaptable for use as reinforcements in composite materials.
The method as described in Japanese Patent Application (OPI) No. 59-199809(1984) employs spinning technique using an inorganic salt-containing aqueous solution such as an aqueous solution of rhodanate. In this case, it is necessary to remove inorganic impurities which cause deterioration in strength after spinning and stretching. Therefore, a complicated washing step is required and as a result, it is undesirable from an industrial point of view. Further, when this acrylic fiber is used for precursors to make carbon fibers, it is necessary to remove inorganic impurities completely since they have an adverse effect on the physical properties of carbon fibers. Therefore, this case also requires complicated washing step for produce satisfactory carbon fibers.
The aforementioned techniques are addressed solely to enhancement of fiber strength and they are not concerned with efforts to improve the overall mechanical characteristics and morphology of acrylic fibers. As is well known, the physical properties of carbon fibers prepared by oxidizing and carbonizing acrylic fibers are closely related to the physical properties of the starting acrylic fiber and in consideration of changes in the chemical structure of high molecular chain during oxidizing and carbonizing, it would be more advantageous for the purpose of improving the physical properties of carbon fibers to improve the morphology of acrylic fibers, rather than their mechanical properties, i.e., strength and modulus.